1. Field of the Invention
The present invention relates to a supply voltage regulator for regulating the supply voltage of a power supply for an electronic system, such as a radio pager or a portable radio phone, provided with a functional device requiring a comparatively large driving current, such as a vibrator or an alarm device, capable of limiting the current to be supplied to the functional device to prevent the drop of the supply voltage of the power supply below a predetermined lower limit voltage so that the electronic system is able to operate stably without dissipating more supply power than needed.
2. Description of the Related Art
Generally, a supply voltage regulator incorporated into a portable communication apparatus, such as a radio pager or a portable radio phone, is a voltage stabilizer for maintaining the output voltage constant regardless of the variation of the input supply voltage applied thereto. The three-terminal voltage stabilizer of the HA178M00 series (Hitachi Ltd.) shown in FIG. 2 is an example of such a supply voltage regulator. Referring to FIG. 2, the voltage stabilizer 20 that receives the output voltage of a battery 21 and provides a fixed voltage comprises a voltage regulating transistor 25, voltage-dividing resistors R1 and R2 for dividing the output voltage of the voltage regulating transistor 25, a load circuit 27 having a large-current-driven functional device connected to the output side of the voltage stabilizer 20, a reference voltage generator 26 which generates a reference voltage, and a differential amplifier 24. The differential amplifier 24 of the voltage stabilizer 20 compares the reference voltage provided by the reference voltage generator 26 and the divider output voltage provided by the voltage dividing circuit consisting of the resistors R1 and R2, and applies a voltage corresponding to the difference between the reference voltage and the divider output voltage to the base of the voltage regulating transistor 25 to regulate the voltage drop across the emitter and the collector of the voltage regulating transistor 25.
Since the battery 21 has an internal resistance 28 of r, a voltage drop of r.multidot.i is produced across the output terminals of the battery 21 when a current i flows through the load circuit 27 and, consequently, Vout=E-i.multidot.r, where Vout is the output voltage of the battery 21 and E is the electromotive force of the battery 21. Accordingly, the greater the current i that flows through the load circuit 27 the lower is the output voltage Vout. Eventually, the output voltage Vout of the battery 21 decreases below a minimum operating voltage of the differential amplifier 24 or the load circuit 27 and, consequently, the differential amplifier 24 or the load circuit 27 stop their operations.
Even if the output voltage Vout of the battery 21 is sufficiently high, there is the possibility that the output voltage Vout is caused to drop instantaneously below the minimum operating voltages of the differential amplifier 24 or the load circuit 27 by a rush current or a surge current, so that the differential amplifier 24 and the load circuit 27 become unstable and an abnormally large current flows through the differential amplifier 24 and the load circuit 27. In such a case, the voltage stabilizer 20 is unable to restore its normal operating state unless the battery 21 is disconnected from the voltage stabilizer 20 to reset the voltage stabilizer 20. These problems in the conventional voltage stabilizer are attributable to the lack of a sensing function capable of sensing the current that flows through the load circuit 27 taking into consideration the internal resistance 28 of the battery 21, and a limiting function capable of limiting the current that flows through the load circuit 27. Accordingly, once the supply voltage V.sub.BAT of the battery 21 decreases below a predetermined lower limit voltage, the electronic system including the voltage stabilizer 20 and the load circuit 27 stops its operation even if the battery 21 has a sufficient capacity.
FIG. 3 shows an electronic system incorporating another known voltage stabilizer 20 proposed to solve the problems in the foregoing voltage stabilizer 20 shown in FIG. 2. As shown in FIG. 3, a voltage detector 32 is connected in parallel to the battery 21 to detect the supply voltage V.sub.BAT of the battery 21, i.e., the voltage across the output terminals of the battery 21. A load circuit 27 requiring a large current is connected through a switching circuit 33 to the output side of the voltage stabilizer 20, and electronic system circuit 31 comprising a CPU, a ROM, a RAM and peripheral circuitry is connected across the output terminals of the battery 21.
In normal operation, the switch circuit 33 is closed to supply a large current to the load circuit 27 to drive the latter and, when the supply voltage V.sub.BAT of the battery 21 drops below a predetermined lower limit voltage, the voltage detector 32 inverts its output to open the switch circuit 33, so that the load circuit 27 is disconnected from the battery 21. Since the load circuit 27 is thus disconnected from the battery 21, the supply voltage V.sub.BAT of the battery 21 can be maintained above the minimum operating voltage of the electronic system circuit 31 to avoid the interruption of the electronic system circuit 31. The supply voltage Vsub of the battery 21 increases gradually to its normal level while the load circuit 27 is disconnected therefrom. However, since the load circuit 27 is disconnected from the battery 21, the functional device of the load circuit 27, such as a vibrator or an alarm, is unable to function when necessary.
Since the foregoing known voltage stabilizer 20 is incapable of limiting the current flowing through the load circuit 27, the supply voltage V.sub.BAT of the battery 21 is reduced by a voltage drop attributable to the internal resistance 28 and, consequently, the electronic system circuit 31, as well as the load circuit 27, stops its operation. Thus, the electronic system including the load circuit 27, such as a portable info-communication apparatus, is unable to function continuously and stably within the life of the battery 21. Therefore, it has been necessary to provide the electronic system with a backup battery in addition to the main battery 21. Operating conditions for the electronic system at a comparatively low temperature is more severe than those at an ordinary temperature because the internal resistance 28 of the battery increases at a comparatively low temperature and a large current flows through the load circuit 27. Accordingly, the operating temperature range of the electronic system is narrowed inevitably. Since the electronic system need the peripheral circuits including the voltage detector 32 and the switch circuit 33 in addition to the voltage stabilizer 20, those peripheral circuits increases the power consumption and the cost of the electronic system.